Scrapie is a spongiform encephalopathy of sheep and goats which can be transmitted experimentally to several other animal species. Similar diseases are recognized in cattle and humans. No etiologic agent has been identified. However, the proteinase K resistant form (PrP-res) of an endogenous protein designated prion protein (PrP) purifies with infectivity and is important to disease pathogenesis. We developed a sensitive assay for PrP-res and utilized it to diagnose scrapie in sheep. Analysis based on PrP-res detection was much more accurate and less subjective than the currently used method of diagnosis based on the microscopic evaluation of brain. We also showed that PrP- res analysis of spleen or lymph node was nearly as accurate as analysis of brain. We have also shown that PrP-res accumulates prior to clinical disease in sheep lymph node and placenta. Thus analysis of sheep placenta or lymph node provides an ante mortem test for infection. We are also using PrP-res analyses to test tissues from cattle in order to determine if spongiform encephalopathy currently exists in U.S. cattle and, thereby, whether an epidemic similar to BSE in Great Britain is possible in the U.S.A. PrP-res analysis should also be relevant for diagnosis of the human disease counterparts. The influence of specific PrP gene sequences on interspecies transmission of spongiform encephalopathies is also being studied. To do so, we have expressed various mouse-hamster PrP constructs in scrapie-infected mouse neuroblastoma ( MNB ) cells and are now analyzing them in mice and hamsters to determine if species tropism has been altered. Mouse neuroblastoma cells are also being used to study the normal function of the PrP protien as well as to identify factors which might account for the biochemical changes which lead to the conversion of the endogenous PrP protein to the disease associated PrP-res form. Similar experiments are being done in vivo using transgenic mice containing the hamster PrP gene.